Efficient mediatorless superoxide sensors using cytochrome c-modified electrodes: surface nano-organization for selectivity and controlled peroxidaseactivity

Amperometric superoxide anion sensor electrodes were prepared by immobilizi ng cytochrome c (Cyt c) on COOH-terminated Au-alkanethiolate monolayers. Mo nolayers and mixed-monolayers of 3-mercaptopropionic acid (MPA) with the co adsorbate, 3-mercaptopropanol (MP), were constructed on the surface of Au e lectrodes. The electrochemical characteristics and the superoxide anion sen sor activities of cytochrome c immobilized on the monolayers and mixed-mono layers of MPA were found to depend largely on the structure of the underlyi ng alkanethiolate monolayer. While cytochrome c on a MPA monolayer, Au/MPA/ Cyt c, showed a reversible redox wave at 0.08 V (vs. Ag \ AgCl \ NaCl (sat. )), cytochrome c on the mixed-monolayer of MPA and MP, Au/MPA + MP/Cyt c, s howed a wave at - 0.01 V. Cytochrome c immobilized on the Au-alkanethiolate layer was supposed to acquire different conformations at each of the Au/MP A/Cyt c and Au/MPA Jr MP/Cyt c electrodes, leading to a difference in the t hermodynamic potential of cytochrome c. The apparent heterogeneous electron -transfer rate constant, k(het), is determined by fast-scan cyclic voltamme try to be (2.8 +/- 0.4) x 10(3) s(-1) for the Au/MPA + MP/Cyt c electrode. Both Au/MPA/Cyt c and Au/MPA + MP/Cyt c electrodes show anodic currents to O-2(-) anion generated by enzymatic reaction with a response time of simila r to 15 s, and the magnitude of the current response is higher at Au/MPA MP/Cyt c relative to that at Au/MPA/Cyt c. While H2O2 did not give any curr ent response at the Au/MPA electrode at the applied potential of 0.15 V, cy tochrome c immobilized on the Au-alkanethiolate layers was found to reduce H2O2; the cathodic current recorded for the reduction of H2O2 is very much higher at Au/MPA/Cyt c than that at Au/MPA + MP/Cyt c at all the applied po tentials. The O-2(-) anion sensor activity of the two electrodes was compar ed with that of Au/HS(CH2)(10)COOH/Cyt c. Among the cytochrome c-modified e lectrodes tested, Au/MPA + MP/Cyt c showed a high anodic current for O-2(-) anion and low interferences, 17 and 7%, due to the electrochemical interfe rents, H2O2 and uric acid, respectively. (C) 2000 Elsevier Science S.A. All rights reserved.